Turbomachines such as gas turbine engines have one or more turbine modules each of which includes a plurality of blades and vanes for exchanging energy with a working medium fluid. Because the blades and vanes of a turbine module operate in a hostile, high temperature environment, they are typically made of a high strength, temperature tolerant substrate material and coated with a damage resistant protective coating. Despite the presence of the protective coating, the blades and vanes are nevertheless susceptible to localized burning, cracking, erosion, sulfidation, oxidation and other types of damage. The damage may be confined to the coating, or may breach the coating and extend into the substrate.
When it is necessary to repair a locally damaged component, such as a blade or vane, it is common practice to first remove the damaged component from the turbine module. The damaged component is repaired by first abrading away the locally damaged material and applying a coating precursor to the abraded region. The component is then placed in a diffusion furnace and the furnace is flooded with a nonreactive gas such as argon. The component is heated to a predefined temperature for a predetermined time so that the coating precursor diffuses into the component. The repaired component is subsequently reinstalled in the turbine module and the module is returned to service.
Although the above described repair procedure is highly effective, the preliminary step of removing the damaged component from its parent module often entails substantial disassembly of the module, making the repair inordinately time consuming and expensive. Nevertheless, module disassembly and component removal is a widely accepted practice because an entire turbine module is far too large to fit into most commercially available diffusion furnaces. Even if a large enough furnace were available, the sheer weight and bulk of a turbine module would complicate the task of moving the module into and out of the furnace.
The expense and delay of module disassembly may be avoided by conducting an in-situ repair of localized damage to turbine blades and vanes. In-situ repair is feasible when the damaged component is reasonably accessible to a repair technician without removing the component from the module. An existing in-situ repair method is similar to is the method describe above except that the diffusion step is dispensed with. Instead, the in-situ method relies on normal engine operational temperatures to diffuse the coating precursor into the damaged component after the module has been returned to service. Experience has shown, however, that the durability of the resultant coating may be unsatisfactory because the engine operational temperatures are too low and too transient to thoroughly diffuse the coating precursor into the damaged component. Moreover, attempted diffusion of the precursor takes place in an oxygen bearing atmosphere, rather than in an inert atmosphere, further jeopardizing the durability of the resultant coating.
In view of these shortcomings, an improved method of repairing localized damage to blades, vanes and other turbomachinery module components is sought.